The Windows 10 license is only 'valid for the lifetime of the device' if you got the upgrade from an OEM machine - in MS-speak, the 'device' is the motherboard.
Changing the motherboard will therefore invalidate the license.
The reason that the Store tells you the system is genuine is because you're still inside the grace period - as soon as you run out of grace, you'll find that message changing.
Please repost your Windows 10 query to the appropriate WIndows 10 Community forum here...
http://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install-winactivate